go 58 lines · 8 steps

Custom query validation in Gin

Bind a search query in Gin, register a custom slice-length validator, and normalize incoming tags before validating.

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Gin Intermediate validation query-binding deduplication custom-validators
1package handlers
2 
3import (
4 "net/http"
5 "strings"
6 
7 "github.com/gin-gonic/gin"
8 "github.com/gin-gonic/gin/binding"
9 "github.com/go-playground/validator/v10"
10)
11 
12type SearchQuery struct {
13 Keyword string `form:"q"`
14 Tags []string `form:"tags" binding:"tagslice,dive,alphanum"`
15 Page int `form:"page,default=1" binding:"min=1"`
16}
17 
18func RegisterTagValidators() {
19 v, ok := binding.Validator.Engine().(*validator.Validate)
20 if !ok {
21 return
22 }
23 v.RegisterValidation("tagslice", func(fl validator.FieldLevel) bool {
24 return fl.Field().Len() <= 10
25 })
26}
27 
28func splitTags(raw []string) []string {
29 seen := make(map[string]struct{})
30 out := make([]string, 0, len(raw))
31 for _, chunk := range raw {
32 for _, part := range strings.Split(chunk, ",") {
33 part = strings.ToLower(strings.TrimSpace(part))
34 if part == "" {
35 continue
36 }
37 if _, dup := seen[part]; dup {
38 continue
39 }
40 seen[part] = struct{}{}
41 out = append(out, part)
42 }
43 }
44 return out
45}
46 
47func SearchArticles(c *gin.Context) {
48 var q SearchQuery
49 q.Tags = splitTags(c.QueryArray("tags"))
50 
51 if err := c.ShouldBindQuery(&q); err != nil {
52 c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
53 return
54 }
55 
56 articles := articleRepo.Search(c.Request.Context(), q.Keyword, q.Tags, q.Page)
57 c.JSON(http.StatusOK, gin.H{"data": articles, "tags": q.Tags})
58}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Gin exposes its underlying validator engine so you can register domain-specific rules by name.
  2. 2Struct tags let a single field chain multiple constraints like a custom rule plus per-element checks.
  3. 3Normalizing input before binding keeps validation logic focused on rules rather than cleanup.

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